PURPOSE: To make annunciation easy to understand regardless of application pattern of a detector, by operating a display element and a buzzer when a member is detected as present behind a wall or the like.

CONSTITUTION: A back member is present on a wall and as the electrostatic capacitance detected by an electrode 2 grows, an output pulse width of a mono-stable multivibrator 21 increases. On the other hand, time constant of a mono-stable multivibrator 22 is set with a reference time constant circuit and a pulse with the pulse width corresponding to the time constant is outputted. Outputs of both the mono-stable multivibrators 21 and 22 are inputted into a time difference detection circuit 23 and a pulse width is outputted as obtained by subtracting an output pulse width of the mono-stable multivibrator 22 from an output pulse width of the mono-stable multivibrator 21. Thus, as the output pulse width of the mono-stable multivibrator 21 grows, an output pulse width of the circuit 23, namely, an integration circuit 24 increases. A switching circuit 25 has a plurality of thresholds, and with an increase in the output level of the circuit 24, light emitting diodes LD4WLD1 come ON sequentially and with the lighting of the diode LD1, a buzzer BZ1 connected in parallel sounds simultaneously. This enables the checking of the presence of the back member by a sound alarm depending on an application pattern even when the lighting of a display element is not visible.